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8th grade (U.S.)

8th grade is all about tackling the meat of algebra and getting exposure to some of the foundational concepts in geometry. If you get this stuff (and you should because you're incredibly persistent), the rest of your life will be easy. Okay, maybe not your whole life, but at least your mathematical life. Seriously, if you really get the equations and functions stuff we cover here, most of high school will feel intuitive, even relaxing.
(Content was selected for this grade level based on a typical curriculum in the United States.)

In 72 years, Sal will be 3 times as old as he is today (although he might not be... um... capable of doing much). How old is Sal today?
These classic questions have plagued philosophers through the ages. Actually, they haven't. But they have plagued algebra students! Even though few people ask questions like this in the real-world, these are strangely enjoyable problems.